Optical scanners are widely used in laser equipment to digitize images on reflective or transmissive media, or to print images on reflective or transmissive media. In the former application, the laser beam is unmodulated, while in the latter application, the laser beam is modulated with information. One application of an optical scanner is to create depth or motion images on lenticular material having a recording layer. Spatially multiplexed images are exposed onto the recording layer by a laser optical scanner.
It has been standard practice in the art of optical scanner design to correct the system for defects in the manufacturing of the optical scanning device. Such devices are usually rotating polygons, but they can also be galvanometers. The typical defect that occurs in fabricating polygons is called "pyramid error." This occurs when there is a difference in angle between polygon facets in the cross scan axis.
It is common in the art to conjugate the cross scan axis of the polygon facet to the image plane with a cylinder mirror. Since the laser spot must also be conjugated to the image plane, the laser must be focused at a certain spot size on the polygon facet. The size of the spot on the polygon facet depends on the desired spot size at the final image plane and the magnification of the conjugating cylinder mirror. If the required spot size at the image plane is small (i.e., a high resolution scanner) that will then require either a small spot on the polygon facet or a larger spot on the polygon facet with a high reducing magnification of the conjugating cylinder. If the spot on the polygon is small, artifacts in the image will be produced through diffraction of the light off of tooling marks and dust on the polygon facet. So, there is a desire to have the conjugating cylinder operate a reducing magnification less than 1.times. which will reduce the size of the image of the spot on the polygon. However, as the magnification of the conjugating cylinder departs from 1.times. it introduces coma into the image. This is the dilemma for the optical designer. The desire to have a small spot at the image plane requires either a small spot on the polygon facet (thus producing diffraction artifacts) or a high reducing magnification in the conjugating cylinder (thus producing coma) which actually enlarges the spot. This effect is also compounded by the incident angle that the light strikes the cylinder mirror. If the mirror reflects the light directly back towards its source, the image will have no coma, however, the image will not be accessible. If, on the other hand, the mirror turns the light at some angle so that the image is accessible, the off-axis image will exhibit coma.
FIG. 1a illustrates how a point source 10 imaged by the curved surface of a cylinder mirror 20 on axis (i.e., an angle of incidence of 0 degrees) is imaged to a coma-free image point 30.
FIG. 1b illustrates how a point source 10 imaged by the curved surface of a cylinder mirror 20 tilted by an angle 40 with respect to the optical axis produces an image 30 aberrated by a certain amount of coma 50. The amount of coma depends upon the radius of curvature of the surface, the distance between the object and the mirror surface, the distance between the mirror surface and the image, the angle of tilt of the mirror with respect to the optical axis, and the numerical aperture of the light. When the object distance and image distance are equal, the magnification is 1 and there is no coma.
The following patents disclose various techniques for correcting optical errors that do not satisfactorily solve these problems.
U.S. Pat. No. 4,759,593, issued Jul. 26, 1988, inventor Kessler.
U.S. Pat. No. 5,768,001, issued Jun. 16, 1998, inventors Kelley et al.
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U.S. Pat. No. 4,832,429, issued May 23, 1989, inventor Nagler.
U.S. Pat. No. 5,267,057, issued Nov. 30, 1993, inventor Sasada.
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U.S. Pat. No. 3,897,132, issued Jul. 29, 1975, inventors Meeussen et al.